An optical wireless communication system wirelessly transmits a light signal generated from a light transmitter to a terminal at the other side. In this system, a light source capable of creating light in a near infrared wavelength band or a white LED capable of creating white light may be used as the light transmitter.
The above-described white LED may also provide lighting features by using white light for data modulation. A white light source made by a combination of LEDs of the three primary colors or an LED in which yellow phosphor is coated on an active layer capable of generating blue light can be used as the light transmitter.
As compared to the light transmitter made by the LEDs of the three primary colors, the white LED in which the yellow phosphor is coated on the active layer capable of generating the blue light may be manufactured cost-effectively and therefore used widely.
However, since the above-described white light source (the white LED in which the yellow phosphor is coated on the active layer capable of generating the blue light) has a slow decay rate to an impulse response of the yellow phosphor, there is a problem in that an available frequency band is narrowed.
FIG. 1 is a graph illustrating the wavelength spectrum of a conventional white LED. Referring to FIG. 1, blue wavelength light has a wavelength bandwidth of about 25 nm at the center of 440 nm (where a transmission coefficient is 0.4) and yellow wavelength light generated from yellow phosphor has a wavelength bandwidth of about 50 nm at the center of 550 nm (where the transmission coefficient is 0.4). It can be seen that the yellow wavelength light has a wavelength bandwidth at least twice as wide as the blue wavelength light.
FIG. 2 is a graph illustrating response characteristics with respect to frequencies of yellow light and white light generated from a white LED. In FIG. 2, the x-axis represents a frequency band (Hz) of a generated light signal and the y-axis represents a gain (dB). Referring to FIG. 2, the bandwidth of blue wavelength light generated from an active layer among lights constituting the white light output from the white LED is 13 MHz and the bandwidth of yellow light generated from a fluorescent material is 2.3 MHz. In the graph of FIG. 2, the dotted line indicates a response of blue wavelength light and the solid line indicates a response of white wavelength light.